Python 3 installation on Linux causes build errors with Python plugin enabled

[peterclemenko]

Migrated here from: realXtend/tundra#663

Having Python 3 installed on Linux will cause the following build errors:

In file included from /usr/include/python3.2mu/Python.h:67:0,
                 from /home/username/dev/rex/naali-deps/install/include/PythonQtInstanceWrapper.h:45,
                 from /home/username/dev/rex/naali-deps/install/include/PythonQt.h:46,
                 from /home/username/dev/rex/naali/src/Application/PythonScriptModule/StableHeaders.h:10,
                 from /home/username/dev/rex/naali/src/Application/PythonScriptModule/PythonScriptModule.cpp:3:
/usr/include/python3.2mu/object.h:402:18: error: expected unqualified-id before ‘;’ token